POSSIBLE INVOLVEMENT OF RESIDENT HYDROPHOBIC THIOLS OF SULFHYDRYL PROTEINS IN THE GLUTATHIONE PEROXIDASE MIMICRY OF DIPHENYL DISELENIDE

Abstract

There is a strong line of evidence speculating that the translation of the chemical model of the glutathione peroxidase (GPx) mimetic catalytic cycle of organoseleniums into simple biological model may be integrally linked with their oxidation of critical thiols on proteins of biological importance. However, such hypothesis is devoid of experimental validation. Herein, the GPx mimicry-linked antioxidant action of the lipophilic organoselenium diphenyl diselenide [DPDSe] was evaluated against cerebral and hepatic lipid peroxidation in the presence and absence of the hydrophobic thiol modifier, N-(-4-Acetylphenyl)-2-Iodoacetamide [APIAM], with or without prooxidants [Fe2+ (10 μM), sodium nitroprusside (5 μM), H2O2 (2 mM) 3-nitropionic acid (3-NPA, 1 mM), and quinolinic acid (QA, 1 mM). In addition, the influence of both APIAM and DPDSe on the oxidation of mono- [cysteine and glutathione] and dithiols [dithiothreitol] was also evaluated. Furthermore, the possible involvement of the catalytically relevant hydrophobic thiols of both the synaptosomal electrogenic transmembrane sodium pump and the cytosolic heme-biosynthetic enzyme, δ-aminolevulinic acid dehydratase [δ-ALAD] in the GPx-like antioxidant mimicry of DPDSe was also determined. The results showed that the antioxidant potency of DPDSe was diminished in the presence of APIAM in either basal or prooxidants-mediated production of aldehydic products of lipid peroxidation in both cerebral and hepatic tissue homogenates. Moreso, both DPDSe and APIAM profoundly oxidized both mono- and dithiols and this effect was markedly noticeable with increasing time. Similarly, both DPDSe and APIAM inhibited the sulfhydryl enzymes, Na+/K+-ATPase and δ-ALAD by a mechanism that is related to oxidation of the catalytically relevant thiols of the enzymes. Interestingly, DPDSe diminished both the basal and prooxidants-provoked formation of lipid peroxidation products with concomitant decrease in the activity of the synaptosomal electrogenic sodium transporter and cytosolic δ-ALAD and this antioxidant effect was hindered in the presence of APIAM. Consequently, considering the fact that both lipophilic compounds [DPDSe and APIAM] oxidize thiols, inhibited the activity of sulfhydryl proteins and that APIAM counteracted the antioxidant action of DPDSe, which is largely centered on its thiol-dependent GPx mimicry, it is rational to strongly conclude that DPDSe may be utilizing the thiols of these sulfhydryl proteins to generate its selenol derivative to effect its GPx-mimetic catalytic cycle. Moreso, the lipophilic nature of both DPDSe and the thiol modifier suggests that both compounds possibly compete with the sulfhydryl group located in the hydrophobic region of the proteins. Hence, this study establishes a further link between the dependence of the pharmacology of DPDSe on its toxicity in simple biological model.